Lubricant compositions comprising colloidal dispersions of rare earth compounds and catalytic combustion of engine exhaust soots therewith

ABSTRACT

Engines equipped with an exhaust line fitted with a particulate filter, in which the particulates contained in the exhaust gas are trapped on such filter, and wherein the trapped particulates are periodically burned off, are operated by improvedly catalyzing the combustion of said particles utilizing a composition containing mixture of a lubricating oil and a colloidal dispersion as the engine-lubricating composition, which dispersion includes particles of at least one compound of at least one rare earth and an amphiphilic agent.

The present invention relates to a method of operating an engine using a lubricating composition comprising a colloidal dispersion of a rare earth to catalyze the combustion of soot.

It is known that during the combustion of diesel fuel in a diesel engine, the carbonated products have a tendency to form soot, which is reputedly harmful both for the environment and for the health. Techniques have long been sought that make it possible to reduce the emission of this soot or carbonated particulates. The same problem is faced for petrol engines that operate in lean burn mode (lean-burn engines) which themselves also emit such particulates.

One satisfactory solution that is now widely used consists in collecting the particulates on a filter which is regularly regenerated to prevent it from clogging up. The regeneration of the filter is facilitated even more when the auto-ignition temperature of the soot is low which may be obtained by introducing a catalyst into the very heart of the soot during the combustion. This technology, known under the name “Fuel-Borne Catalysis” or FBC is also widely used. The thus additivated soot has an auto-ignition temperature that is low enough to be frequently attained during normal running of the engine or during specific regeneration cycles.

Although the FBC technology is satisfactory, there is however a need for other alternative technologies so as to be able to have the widest possible range of solutions and to thus be able to solve the problem of reducing the emission of harmful particulates regardless of the conditions under which this problem is faced.

The object of the invention is therefore to provide one such novel technology.

For this purpose, the invention relates to a method of operating an engine capable of producing exhaust gases that contain particulates and equipped with a muffler provided with a particulate filter, in which the particulates are trapped on said filter and the combustion of the trapped particulates is carried out periodically and which is characterized in that, with a view to catalyzing the combustion of said particulates, use of made, as a lubricating composition for the engine, of a composition which results from the mixture of:

-   -   a lubricating oil; and     -   a colloidal dispersion which comprises particles of a compound         of at least one rare earth and an amphiphilic agent.

The method of the invention has the advantage of doing away with the presence of a specific tank for the soot combustion catalyst and of a device for metering this catalyst into the fuel, unlike the method using FBC technology.

Other features, details and advantages of the invention will appear more completely still on reading the description which follows, and also various concrete but non-limiting examples intended to illustrate it.

The expression “rare earth” is understood to mean, in this same description, the elements of the group composed of yttrium, scandium and the elements from the Periodic Table having an atomic number between 57 and 71 inclusive.

The composition of the invention results from a mixture of two essential components: the lubricating oil and the colloidal dispersion.

Lubricating oils are well known to a person skilled in the art. It may be recalled that these products contain a base oil having a lubricating property. This base oil may be a mineral oil derived from petroleum, especially based on paraffins, aromatics or isoparaffins and mixtures of these compounds. The mineral oil may be obtained by vacuum distillation of a crude oil, the distillate obtained is then hydrocracked, hydrotreated and subsequently dewaxed and/or hydroisomerized so as to improve the properties such as the viscosity and those of the flow of the base oil thus obtained.

These base oils may also be synthesis oils based on polyalphaolefins or organic esters.

The viscosity index of the mineral oils may be, for example, between 90 and 100 (index measured according to the ASTM D2270 standard), that of the hydrotreated products between 120 and 130 and this index may be greater than 140 for synthetic oils based on polyalphaolefins and may even reach 200 for those based on organic esters.

Also in a known manner, the lubricating oils additionally contain various additives that can be classified into three groups: those intended to improve the chemical stability of the oil or to inhibit the effects of degradation products, those which improve the rheological properties and those which protect metallic surfaces and have a wear-resistant effect.

Found in the first group are antioxidant additives based, for example, on phenols, on substituted arylamines or on sulfur-containing compounds or also on zinc dialkyldithiophosphates. Also found are detergent additives of the type of salts of organic acids or of phenols or of divalent metals and dispersant additives of the organic surfactant type.

The additives of the second group are those which act on the pour point of the oils and are of the type of oligomers having alkyl chains or else the products known as antifreeze products of the alkylnaphthalene type, long-chain polyalcohol acrylates, or else of the alkylated polystyrene type. Also found in this second group are additives that improve the viscosity index. These additives are based on hydrocarbon-based polymers (for example, ethylene/propylene copolymers) or on polymers having an ester functional group (of polymethacrylate type). Finally, also found in this second group of additives are anti-foaming products, for example based on silicones.

The third group of additives comprises products with a wear-resistant effect. These are generally organic products containing sulfur, chlorine or phosphorus of the type of dithiophosphoric derivatives or phosphomolybdate derivatives.

The second main component of the composition of the invention is the colloidal dispersion.

The expression “colloidal dispersion” denotes, in the present description, any system composed of fine solid particles of colloidal dimensions based on a compound of a rare earth, in suspension in a liquid phase, said particles possibly, in addition, optionally containing residual amounts of bonded or adsorbed ions such as, for example, nitrates, acetates, citrates or ammoniums. The expression “colloidal dimensions” is understood to mean dimensions between around 1 nm and around 500 nm.

The particles may more particularly have an average size of at most about 250 nm, especially of at most 100 nm, preferably of at most 20 nm and more preferably still of at most 15 nm. It will be noted that in such dispersions, the rare-earth compound may be, preferably, completely in the form of colloids, or in the form of colloids and partially in the form of ions.

The particle size distribution, of which mention is made above and for the remainder of the description, except where indicated otherwise, is determined by transmission electron microscopy (TEM), conventionally, on a sample that has first been dried and deposited on a carbon membrane supported on a copper grid.

The rare earth may be chosen, more particularly, from cerium, lanthanum, yttrium, neodymium, gadolinium or praseodymium. Cerium may most particularly be chosen.

According to a first variant of the invention, the colloidal dispersion is characterized in that it comprises particles of a compound of cerium and of another rare earth.

According to another variant, the colloidal dispersion of the invention is characterized in that it comprises particles based on a compound of cerium, optionally of another rare earth and of iron.

More particularly, when the particles of the dispersion of the invention are based on a compound of several elements, that is to say cerium, another rare earth and/or iron, these elements are in a mixture within each particle, these elements generally being in the form of mixed oxides and/or hydrated mixed oxides (oxyhydroxides).

In the case of a colloidal dispersion of cerium, this element is preferably mainly in the form of cerium IV. For example, the content of cerium III relative to cerium IV (content expressed by the Ce III/total Ce atomic ratio) is generally at most 40%. It may vary as a function of the embodiments of the dispersions used and may thus be at most 20%, more particularly at most 10%, and more particularly still at most 1%.

In the case of the aforementioned first variant, the rare earth other than cerium may more particularly be lanthanum or praseodymium. Of course, the present invention covers the case where the particle is a compound of cerium and of several other rare earths in combination.

The proportion of the rare earth other than cerium is preferably at least 10%, more particularly at least 20%, and more particularly still at most 50%, in moles relative to the total number of moles of cerium and of rare earth expressed as oxide.

In the case of the second variant, the proportion of cerium is preferably at most 50%, more particularly at most 20% and more particularly still at most 10%, this proportion being expressed in moles of cerium oxide CeO₂ relative to the total number of moles of cerium oxide and of iron oxide Fe₂O₃.

The two variants may be combined, that is to say that the particles may be compounds of cerium, of at least one other rare earth and of iron.

As indicated above, the particles of the colloidal dispersion are in suspension in a liquid phase which here is an organic phase.

This organic phase may be composed of the base oil having a lubricating property described above or it may also be a mixture of this base oil with another organic phase, miscible with this oil. Specifically and as will be seen later on, the lubricating composition of the invention may be obtained by mixing the lubricating oil with a previously prepared colloidal dispersion. In this case, this dispersion comprises an organic phase which may be a hydrocarbon, more particularly an apolar hydrocarbon.

By way of example of an organic phase, mention may be made of the aliphatic hydrocarbons such as hexane, heptane, octane, nonane, inert cycloaliphatic hydrocarbons such as cyclohexane, cyclopentane and cycloheptane, aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes and liquid naphthenes. Also suitable are oil cuts of the ISOPAR or SOLVESSO type (trade marks of Exxon), especially SOLVESSO 100 which mainly contains a mixture of methylethylbenzene and trimethylbenzene, SOLVESSO 150 which contains a mixture of alkylbenzenes, in particular of dimethylbenzene and tetramethylbenzene and ISOPAR which mainly contains C-11 and C12 isoparaffinic and cycloparaffinic hydrocarbons. Mention may also be made, as other oil cuts, of those of the PETROLINK® type from Petrolink or of the ISANE® type from Total.

Use may also be made, for the organic phase, of chlorinated hydrocarbons such as chlorobenzene or dichlorobenzene, or chlorotoluene. Ethers and also aliphatic and cycloaliphatic ketones such as, for example, diisopropyl ether, dibutyl ether, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone and mesityl oxide, may be envisioned.

Esters may be envisioned, but they have the drawback of risking being hydrolyzed. Mention may be made, as esters capable of being used, of those derived from the reaction of acids with C1 to C8 alcohols and especially the palmitates of secondary alcohols such as isopropanol. Mention may be made of butyl acetate by way of example.

Of course, the organic phase may be based on a mixture of two or more hydrocarbons or compounds of the type described above.

Finally, as indicated above, the choice of the organic phase from the examples which have just been given will be made depending on its compatibility or miscibility with the lubricating oil.

Furthermore, the colloidal dispersion comprises an amphiphilic agent.

This amphiphilic agent at least partly interacts, either via grafting, or via electrostatic bonding, with the particles of the rare earth and, optionally, iron compound.

This agent may more particularly be an acid.

The acid is more particularly chosen from organic acids which comprise at least 6 carbon atoms, more particularly still from 10 to 60 carbon atoms, preferably from 10 to 50 carbon atoms and more preferably still from 15 to 25 carbon atoms.

These acids may be linear or branched. They may be aryl, aliphatic or arylaliphatic acids, optionally bearing other functional groups on condition that these functional groups are stable in the media where it is desired to use the dispersions according to the present invention. Thus, use may be made, for example, of aliphatic carboxylic acids, aliphatic sulfonic acids, aliphatic phosphonic acids, alkoylarylphosphonic acids and alkoylarylsulfonic acids that have around 10 to around 40 carbon atoms, which are natural or synthetic. It is of course possible to use the acids as a mixture.

It is also possible to use carboxylic acids where the carbon-based chain bears ketone functional groups such as pyruvic acids substituted at the alpha position with the ketone functional group. These may also be α-halo-carboxylic acids or α-hydroxycarboxylic acids. The chain attached to the carboxylic group may bear unsaturations. The chain may be interrupted by ether or ester functional groups on condition that the lipophilicity of the chain bearing the carboxylic group is not impaired too much.

By way of example, mention may be made of the fatty acids of tall oil, of soybean oil, of tallow or of linseed oil, oleic acid, linoleic acid, stearic acid and isomers thereof, pelargonic acid, capric acid, lauric acid, myristic acid, dodecylbenzenesulfonic acid, 2-ethylhexanoic acid, naphthenic acid, hexoic acid, toluenesulfonic acid, toluenephosphonic acid, laurylsulfonic acid, laurylphosphonic acid, palmityl-sulfonic acid and palmitylphosphonic acid.

As an amphiphilic agent, mention may also be made of polyoxyethylenated alkyl ether phosphates. These are understood here to be organophosphates of formula:

or else polyoxyethylenated dialkoyl phosphates of formula:

in which:

-   -   R¹, R², R³, which are identical or different, represent a linear         or branched alkyl radical, especially having 2 to 20 carbon         atoms; a phenyl radical; an alkylaryl radical, more particularly         an alkylphenyl radical, especially with an alkyl chain having 8         to 12 carbon atoms; an arylalkyl radical, more particularly a         phenylaryl radical;     -   n is the number of ethylene oxide units which may range from 0         to 12 for example; and     -   M represents a hydrogen, sodium or potassium atom.

The radical R¹ may especially be a hexyl, octyl, decyl, dodecyl, oleyl or nonylphenyl radical.

Mention may be made, as an example of this type of amphiphilic compounds, of those sold under the trade marks LUBROPHOS® and RHODAFAC® sold by Rhodia and especially the products below:

-   -   the RHODAFAC® RA 600 polyoxyethylene (C8-C10) alkyl ether         phosphates;     -   the RHODAFAC® RS 710 or RS 410 polyoxyethylene tridecyl ether         phosphate;     -   the RHODAFAC® PA 35 polyoxyethylene oleocetyl ether phosphate;     -   the RHODAFAC® PA 17 polyoxyethylene nonylphenyl ether phosphate;         and     -   the RHODAFAC® RE 610 polyoxyethylene (branched)nonyl ether         phosphate.

The amount of amphiphilic agent present in the dispersion may be defined by the molar ratio r:

r=number of moles of amphiphilic agent/number of moles of compound E E here denoting the rare earth or rare earths or the combination of rare earth(s) and iron.

This molar ratio may be between 0.2 and 1, preferably between 0.4 and 0.8.

The dispersion that can be used in the method of the invention may be according to one specific embodiment.

According to this embodiment, the dispersion is such that at least 90% of the particles are single-crystal particles. The expression “single-crystal particles” is understood to mean particles which, when the dispersion is examined by TEM (high-resolution transmission electron microscopy), appear individualized and composed of a single crystallite.

The cryo-TEM technique may also be used to determine the aggregation state of the elementary particles. This technique makes it possible to observe, via transmission electron microscopy (TEM), samples that are kept frozen in their natural medium which is either water or organic diluents such as aromatic or aliphatic solvents such as, for example, SOLVESSO and ISOPAR or else certain alcohols such as ethanol.

Freezing is carried out on thin films of around to 50 100 nm in thickness either in liquid ethane for aqueous samples or in liquid nitrogen for others.

With cryo-TEM, the dispersion state of the particles is well preserved and representative of that present in the actual medium.

According to this embodiment, the particles have a fine and narrow particle size distribution. Specifically, they have a D₅₀ between 1 and 5 nm, preferably between 2 and 3 nm.

Generally, and by way of example only, the concentration of the dispersion of rare earth, and optionally of iron, is between 1 and 40% by weight of oxide(s) of rare earth(s) or of oxide(s) of rare earth(s) and of iron Fe₂O₃ relative to the total weight of the dispersion.

Finally, generally and by way of example for the colloidal dispersions of a compound of a rare earth, optionally of a rare earth and of iron and preparation thereof, reference may be made to the whole of the description of each of Patent Applications EP 0 671 205, WO 01/10545 and WO 97/19022.

The lubricating composition that can be used in the method of the invention may be prepared by mixing a lubricating oil with a colloidal dispersion of a rare-earth compound or of a rare-earth and iron compound. This mixing may be carried out in proportions which are not critical and which may vary over a wide range. By way of example, these proportions may be such that the content of rare earth, optionally of rare earth and of iron, in the lubricating composition, expressed as metal element, and originating from the colloidal dispersion is at most 15% by weight of the total composition, more particularly at most 10%, proportions of a few percent only being possible.

It is observed that the composition thus obtained is stable, that is to say that no settling of the dispersion and therefore no deposition of the cerium or iron particles at the bottom of the tank containing the lubricating composition is observed. Furthermore, this stability is maintained even when the lubricating composition is exposed to a high temperature, which is the case during the operation of the engine for which the composition is used as a lubricant. Moreover, it is unexpectedly observed that the use of this composition in the operation of the engine definitely leads to a catalysis of the combustion of soot.

The method of the invention applies to an engine which, during its operation, is capable of producing harmful particulates, such as soot, which are found in the exhaust gases. It may more particularly be a diesel or a petrol engine operating in lean burn mode.

This method applies to an engine which, in a known manner, is equipped with an exhaust line or muffler, integrated into which is a particulate filter. Conventionally, this filter comprises a filter of the type having a filtering wall made of ceramic or made of silicon carbide through which the exhaust gases circulate. However, it may also be one or more screens made of wire mesh or else a foam-type filter made of ceramic or made of fibrous material.

The method of the invention aims to catalyze the combustion of the particulates or soot trapped on the particulate filter. In the case of the invention, the rare-earth or iron and rare-earth compound is used as a catalyst for the combustion of this soot and it is conveyed by the lubricating composition and not by the fuel as in the methods of the prior art. The lubricating composition, prepared prior to its use in the engine and therefore comprising the dispersion of rare earth or of iron and rare earth, is introduced into the oil reservoir of the engine, for example during an oil change. The lubricating composition thus passes into the lubricating circuit of the engine. It is observed that the rare-earth or rare-earth and iron compound introduced by the lubricating composition is found in the soot and may thus help to catalyze the combustion thereof.

Of course, it is possible, without departing from the scope of the present invention, to implement the above method while using, for the operation of the engine, a fuel which furthermore contains a catalyst for the combustion of the soot. This catalyst may also be a colloidal dispersion such as described above. It is similarly possible to implement the method of the invention in a system in which the muffler is equipped with a catalyzed particulate filter. This type of filter is well known, it is a filter in which, during its manufacture, a catalyst for the oxidation of particulates or soot is incorporated.

Examples will now be given.

EXAMPLE 1

This example relates to the preparation of a lubricating composition that can be used in a method according to the invention.

For this preparation, use was made of a colloidal dispersion based on cerium prepared according to a method of the type of that from Example 4 of Patent Application EP 671 205, hut with a temperature of 160° C. for the autoclave treatment. The organic phase of this dispersion was ISOPAR and the amphiphilic agent was isostearic acid. The content of ceric nitrate used was adjusted so as to obtain a colloidal dispersion containing 25% by weight of metal cerium. The colloids had an average size of 5 nm.

Added to this dispersion was a commercial oil (Total Activa Diesel 10W40) so as to obtain a lubricating composition containing 43% by weight of this commercial oil and 57% by weight of the colloidal dispersion.

EXAMPLE 2

This example relates to the preparation of a second lubricating composition that can be used in a method according to the invention.

For this preparation, use was made of a colloidal dispersion based on cerium and on iron in a molar proportion of metal of 50/50, prepared according to a method of the type from Example 3 of Patent Application WO 01/10545. The organic phase of this dispersion was ISOPAR and the amphiphilic agent was isostearic acid. The dispersion contained 10% by weight of metal (cerium and iron). The colloids had a size between 3 and 4 nm and were perfectly individualized.

Added to this dispersion was a commercial oil (Total Activa Diesel 10W40) so as to obtain a lubricating composition containing 28% by weight of this commercial oil and 72% by weight of the colloidal dispersion.

EXAMPLE 3

This example relates to a test for catalytic oxidation of the soot carried out in the presence of a lubricating composition that can be used in a method according to the invention. The catalytic soot oxidation properties were measured by thermogravimetric analysis. A Setaram thermohalance equipped with a quartz boat, in which a specimen containing around 20 mg of sample was placed, was used.

The sample was composed of a mixture of 20% by weight of the lubricating composition from Example 1 and 80% by weight of carbon black. The carbon black used to simulate the soot emitted by a diesel engine was the carbon black sold by Cabot under the reference ELFTEX 125. The lubricating composition and carbon black mixture was homogenized by mixing with a spatula. The paste thus obtained was first dried in a ventilated oven at 60° C. then at 120° C.

20 mg of the sample thus prepared and treated were introduced into the boat of the thermobalance, then a gas stream composed of an air/water mixture in respective volume proportions of 87% and 13% was circulated. After a hold of 30 minutes at 150° C., the temperature rise up to 900° C. was started with a ramp of 10° C./min and the mass loss of the sample was recorded as a function of the temperature.

EXAMPLE 4

This example was carried out according to the same procedure as Example 3 but with the lubricating composition described in Example 2.

COMPARATIVE EXAMPLE 5

This example relates to a soot oxidation test carried out in the presence of a lubricating composition from the prior art. The test was carried out according to the same procedure but using the pure commercial oil Total Activa Diesel 10W40. The sample thus evaluated was therefore composed of a mixture of 20% by weight of the pure commercial oil and 80% by weight of carbon black.

The results are given in table 1: they are expressed as the half-oxidation temperature of the soot (T50% (soot)) corresponding to the temperature required to obtain half of the mass loss measured between 200° C. and 900° C.

TABLE 1 Example T50% (soot) in ° C. 3 510° C. 4 615° C. 5 Comparative 610° C. Soot alone 620° C.

It is observed that the addition of pure commercial oil has no or little effect on the half-conversion temperature of the soot, whereas the use of a lubricating composition containing a commercial oil and a colloidal dispersion containing cerium or cerium and iron makes it possible to very significantly reduce the combustion temperature of the soot.

EXAMPLE 6

In this example the colloidal dispersion of cerium and of iron from Example 2 was taken again and 10 g of this dispersion containing 10% by weight of metal cerium and metal iron were added to 160 g of an engine oil (Elf Prestigrade 15W40) so as to obtain a lubricating composition containing 94% by weight of this commercial oil and 6% by weight of the commercial colloidal dispersion. The content of metal iron of this lubricating composition was thus 0.20% by weight, whereas that of metal cerium was 0.45% by weight.

This lubricating composition was then introduced into a partially stoppered container, that was itself placed in a ventilated chamber kept at 110° C. The iron and cerium content of the composition was then regularly measured in the upper part of the container via a chemical metering technique (ICP).

Table 2 below gives the iron and cerium contents thus measured after various residence times in the chamber at 110° C.

TABLE 2 Heating time at 110° C. in days wt % cerium wt % iron 0 0.45 0.20 1 0.45 0.20 3 0.45 0.20 6 0.45 0.20 10 0.45 0.20 13 0.45 0.20 17 0.45 0.20 28 0.45 0.20 38 0.45 0.20 55 0.45 0.20 79 0.45 0.20

It is therefore observed that the thermal stability of this lubricating composition is very high considering that the iron and cerium contents do not change over 79 days of continuous heating at 110° C. This stability time determined under these conditions may be considered as sufficient to ensure the stability of the lubricating composition between two oil changes of the engine oil circuit. 

1.-12. (canceled)
 13. A method for operating an engine producing exhaust gases that contain particulates and equipped with a muffler provided with a particulate filter, in which the particulates are trapped on said filter and the catalytic combustion of the trapped particulates is carried out periodically, wherein, to assist catalyzing the combustion of said particulates, a lubricant composition which comprises admixture of: a lubricating oil; and a colloidal dispersion of particles of at least one compound of at least one rare earth and an amphiphilic agent which at least partially interacts therewith, is provided for said engine.
 14. The method as defined by claim 13, wherein said at least one rare earth is selected from the group consisting of cerium, lanthanum, yttrium, neodymium, gadolinium and praseodymium.
 15. The method as defined by claim 13, wherein the colloidal dispersion comprises particles of a compound of iron and of a rare earth.
 16. The method as defined by claim 13, said amphiphilic agent comprising an acid.
 17. The method as defined by claim 16, said acid comprising a carboxylic acid having from 10 to 60 carbon atoms.
 18. The method as defined by claim 16, said acid being selected from the group consisting of fatty acids of tall oil, of soybean oil, of tallow or of linseed oil, oleic acid, linoleic acid, stearic acid and isomers thereof, pelargonic acid, capric acid, lauric acid, myristic acid, dodecylbenzenesulfonic acid, 2-ethylhexanoic acid, naphthenic acid, hexoic acid, toluenesulfonic acid, toluenephosphonic acid, laurylsulfonic acid, laurylphosphonic acid, palmitylsulfonic acid and palmitylphosphonic acid.
 19. The method as defined by claim 13, wherein the colloidal dispersion comprises particles of at least one compound of at least one rare earth, at least 90% of such particles being single-crystal particles.
 20. The method as defined by claim 19, wherein the particles have a D₅₀ value ranging from 1 to 5 nm
 21. The method as defined by claim 13, wherein the content of rare earth, optionally of rare earth and of iron, expressed as metal element, comprising the colloidal dispersion in the lubricating composition, is at most 15% by weight of the total weight thereof.
 22. The method as defined by claim 13, utilizing a lubricant composition which comprises the lubricating oil and the colloidal dispersion prepared prior to providing same in the engine.
 23. The method as defined by claim 13, wherein the engine is a diesel engine or a petrol engine.
 24. The method as defined by claim 13, carried out employing an engine operating with a fuel which contains a catalyst for combustion of the particulates or with an engine equipped with a muffler which comprises a catalytic particulate filter.
 25. A lubricant composition which comprises admixture of: a lubricating oil; and a colloidal dispersion of particles of at least one compound of at least one rare earth and an amphiphilic agent which at least partially interacts therewith.
 26. The lubricant composition as defined by claim 25, said amphiphilic agent comprising an acid.
 27. The lubricant composition as defined by claim 25, said colloidal dispersion comprising particles of at least one cerium compound.
 28. The lubricant composition as defined by claim 27, said colloidal dispersion optionally comprising particles of another rare earth and of iron.
 29. The lubricant composition as defined by claim 28, said colloidal dispersion comprising particles of a mixed oxide and/or a hydrated mixed oxide.
 30. The lubricant composition as defined by claim 25, said colloidal dispersion comprising particles of at least one compound of at least one rare earth elected from the group consisting of cerium, lanthanum, yttrium, neodymium, gadolinium and praseodymium.
 31. The lubricant composition as defined by claim 25, said colloidal particles comprising a complexing agent.
 32. The lubricant composition as defined by claim 25, said colloidal particles being suspended in an organic liquid phase.
 33. An internal combustion engine oil comprising the lubricant composition as defined by claim
 25. 